The present invention relates to a dialysis apparatus for independently controlling the concentration of at least two ionic substances inside a patient""s body.
The kidneys fulfill many functions, including the removal of water, the excretion of catabolites (or waste from the metabolism, for example urea and creatinine), the regulation of the concentration of the electrolytes in the blood (sodium, potassium, magnesium, calcium, bicarbonates, phosphates, chlorides) and the regulation of the acid/base equilibrium within the body, which is obtained in particular by the removal of weak acids (phosphates, monosodium acids) and by the production of ammonium salts.
In individuals who have lost the use of their kidneys, since these excretion and regulation mechanisms no longer work, the body accumulates water and waste from the metabolism and exhibits an excess of electrolytes (in particular sodium), as well as, in general, acidosis, the pH of the blood plasma shifting towards 7 (the blood pH normally varies within narrow limits of between 7.35 and 7.45).
In order to overcome renal dysfunction, resort is conventionally made to a blood treatment involving extracorporeal circulation through an exchanger having a semipermeable membrane (haemodialyser) in which the patient""s blood is circulated on one side of the membrane and a dialysis liquid, comprising the main electrolytes of the blood in concentrations close to those in the blood of a healthy subject, is circulated on the other side. Through the effect of the physical phenomenon referred to as dialysis, the molecules migrate from the liquid where their concentration is higher to the liquid where their concentration is lower.
In conventional dialysis apparatus, the dialysis liquid is prepared by metered mixing of water and two concentrated solutions, the first concentrated solution containing sodium chloride and sodium bicarbonate, and the second concentrated solution containing, calcium chloride, potassium chloride and magnesium chloride as well as acetic acid. The function of the acetic acid is to limit the formation of calcium carbonate and magnesium carbonate precipitates which form undesirabledeposits in the hydraulic circuit of the dialysis apparatus.
This conventional way of preparing a dialysis liquid has several drawbacks:
the respective concentrations of the various ionic substances involved in the composition of the dialysis liquid cannot be regulated independently of one another, even though this would be desirable at least for sodium, potassium and bicarbonate;
for physiological reasons, the acetic acid concentration in the dialysis liquid is necessarily limited, so that carbonated deposits are formed in the hydraulic circuit of the dialysis apparatus. Dialysis apparatus therefore need to be regularly descaled, which places a further burden on their maintenance;
the presence of acetic acid in the second solution causes corrosion of the connection and pumping means used for transferring the concentrated solution from the reservoir where it is contained to the mixing zone of the apparatus, where it is diluted in the solution obtained by metered mixing of water and the first solution;
use of a dialysis liquid having a pH less than that of blood at the time when certain types of haemodialysers are started up, that is to say at the time when the dialysis liquid compartment of these haernodialysers is filled with dialysis liquid and the blood compartment is filled with diluted blood, seems to be one of the cofactors of certain hypersensitivity reactions.
Document EP 0 192 588 describes a dialysis apparatus comprising:
means for circulating a dialysis liquid which contains sodium bicarbonate and is free of calcium and magnesium,
means for infusing the patient with an infusion solution containing at least calcium and magnesium.
With this apparatus, since the ionic substances (bicarbonate, calcium, magnesium) which can form precipitates when they are combined, are separated, it is not necessary to involve acetic acid in the composition of the dialysis liquid. As described, however, this apparatus cannot be used because it does not comprise means for regulating the infusion flow rate which, in particular, would take into account the diffusive transport of calcium and magnesium through the membrane of the dialyser, from the patient""s blood to the dialysis liquid. Now, for safety reasons, it is not envisageable to infuse a patient with a"" concentrated calcium solution without being able to regulate the infusion flow rate accurately so that there is neither an excess or deficit of this ionic substance within the patient""s body.
The object of the invention is to provide a dialysis apparatus which makes it possible to regulate accurately the flow rate of an infusion liquid containing an ionic substance with a view to making the concentration of this substance inside the patient""s body tend towards a desired concentration. More broadly, the object of the invention is to provide a dialysis apparatus which makes it possible to control separately the concentration of at least two ionic substances inside a patient""s body by means of a dialysis liquid and an infusion liquid.
This object is achieved by means of dialysis apparatus comprising:
means for circulating a dialysis liquid containing sodium chloride and sodium bicarbonate through a haemodialyser;
means for infusing a patient with at least one solution containing at least one ionic substance A (calcium, magnesium, potassium) absent from the dialysis liquid, the substance A having a determined concentration [A]sol in the infusion solution;
means for determining the actual dialysance D of the haemodialyser for sodium;
means for determining a flow rate Qinf of infusion solution such that the concentration of the substance A inside the patient""s body tends towards a desired concentration [A]des, as a function of the dialysance D, of the concentration [A]sol of the substance A in the infusion solution, and of the desired concentration [A]des;
regulating means for regulating the flow rate of infusion solution;
control means for driving the means for regulating the flow rate of the infusion solution such that this flow rate is substantially equal to the determined flow rate Qinf.
In the dialysis apparatus according to the invention, since the dialysis liquid is free of calcium, magnesium and, optionally, potassium, these substances which are present in the blood migrate by diffusion from the blood to the dialysis liquid in the course of the dialysis session. It is in order to compensate for these diffusive losses that provision is made to infuse the patient with a solution containing these substances. The difficulty resides in the fact that these diffusive losses can vary in the course of a dialysis session lasting several hours, and in the fact that an excess or deficit of potassium, calcium and magnesium in the patient""s blood can lead to serious disorders, in particular cardiac disorders. By virtue of the invention, this difficulty is overcome since the flow rate of the infusion is slaved to the actual dialysance of the treatment system.
Moreover, further to remedying the drawbacks of conventional dialysis apparatus mentioned above, the apparatus according to the invention also has the following advantage: provided that the infusion liquid is injected directly into the patient or, downstream of the haemodialyser, into the extracorporeal blood circulation circuit connecting the patient to the haemodialyser, the ionic calcium in the blood migrates by diffusion into the dialysis liquid since the latter is free of calcium. Now, ionic calcium is involved in the sequence of reactions constituting the coagulation process of blood. The extensive calcium depletion of the blood in the haemodialyser therefore partially inhibits the coagulation process, which makes it possible to reduce the amount of anticoagulant usually injected into the extracorporeal blood circulation circuit to prevent blood from coagulating in it.
According to one characteristic of the invention, the means for determining the infusion flow rate Qinf comprise calculation means for calculating the infusion flow rate Qinf according to the formula:   Qinf  =      Cl    xc3x97                            [          A          ]                ⁢        des                                          [            A            ]                    ⁢          sol                -                              [            A            ]                    ⁢          des                    
where Cl is the clearance of the haemodialyser for the substance A, extrapolated on the basis of the dialysance D for sodium.
According to another characteristic of the invention, the infusion solution contains sodium at a determined concentration [Na+]sol. In this case, the apparatus furthermore includes:
means for preparing the dialysis liquid, comprising means for regulating the sodium concentration of the dialysis liquid;
means for determining the sodium concentration [Na+]dial of the dialysis liquid such that the concentration inside the patient""s body tends towards a desired sodium concentration [Na+]des, as a function of the dialysance D, of the flow rate of the infusion liquid Qinf, of the sodium concentration (Na+]sol of the infusion solution, and of the desired sodium concentration [Na+]des; control means for driving the means for regulating the sodium concentration of the dialysis liquid such that this concentration is equal to the determined concentration [Na+]dial.
According to one characteristic of the invention, the means for determining the sodium concentration [Na+]dial of the dialysis liquid comprise calculation means for calculating this concentration according to the formula:             [      "AutoRightMatch"        ⁢    Na    ⁢          +        ⁢          "AutoLeftMatch"      ]        ⁢    dial    =                    Qinf        D            ⁢              xe2x80x83            ⁢                        (          "AutoRightMatch"                [        "AutoRightMatch"            ⁢      Na      ⁢              +            ⁢              "AutoLeftMatch"        ]            ⁢      des        -                  [        "AutoRightMatch"            ⁢      Na      ⁢              +            ⁢              "AutoLeftMatch"        ]            ⁢      sol      ⁢              "AutoLeftMatch"        )              +                  [        "AutoRightMatch"            ⁢      Na      ⁢              +            ⁢              "AutoLeftMatch"        ]            ⁢      des      
Other characteristics and advantages of the invention will become more clearly apparent on reading the following description.